Demand for wireless data is expected to increase significantly in the near term due to the popularity of smart phones and other wireless devices. To meet this growing data demand, one solution is to deploy small cells in areas where such demand exists. Due to the small footprint of small cells, the same frequency can be reused more often in a given area compared to a macro-cell, thus giving more system capacity over a given area.
A cell corresponds to a coverage area provided by a wireless access node. A coverage area can refer to a region where UEs can be provided by a wireless access node with services to a target level. A wireless access node can refer to an active electronic device that is capable of sending, receiving, and forwarding information over a communication channel, and of performing designated tasks. A wireless access node is responsible for performing wireless transmissions and receptions with UEs. Cell and wireless access node are used interchangeably in places where there is no ambiguity. In one embodiment, a UE may, while under the coverage of a small cell and a macro cell, be served by both the macro cell and the small cell. In this case of dual radio connections, the macro cell may provide all control plane functions while small cell provides the bulk of the user plane functions for the dual-connection capable UE.
Control plane functions involve exchanging certain control signaling between a wireless access node and a UE to perform specified control tasks, such as any or some combination of the following: network attachment of a UE, authentication of the UE, setting up radio bearers for the UE, mobility management to manage mobility of the UE (mobility management includes at least determining which infrastructure network nodes will create, maintain or drop uplink and downlink connections carrying control or user plane information as a UE moves about in a geographic area), performance of a handover decision based on neighbor cell measurements sent by the UE, transmission of a paging message to the UE, broadcasting of system information, control of UE measurement reporting, and so forth. Although examples of control tasks and control messages in a control plane are listed above, it is noted that in other examples, other types of control messages and control tasks can be provided. More generally, the control plane can perform call control and connection control functions, and can provide messaging for setting up calls or connections, supervising calls or connections, and releasing calls or connections.
User plane functions relate to communicating traffic data (e.g. voice data, user data, application data, etc.) between the UE and a wireless access network node. User plane functions can also include exchanging control messages between a wireless access network node and a UE associated with communicating the traffic data, flow control, error recovery, and so forth.
The small cell connection can be added or removed from a UE under the control of the macro-cell. For UEs capable of dual connections, the macro cell may be considered to be a primary cell which provides control layer functions visible to the enhanced packet core (EPC) while the small cell acts as a secondary cell for data offload.
However, if the data is offloaded logically between the packet data convergence protocol (PDCP) and the radio link control (RLC) layer then, in an acknowledged mode, the RLC layer will need to acknowledge the PDCP packet data units (PDUs) and this will require communication between the small cell and the macro cell. Such acknowledgements over the backhaul may create significant backhaul traffic and further, latency on the backhaul may cause timeouts in certain circumstances. Similar issues exist when the offloading is performed with radio link control (RLC) layer PDUs.